1. Field of the Invention
The present invention relates to an image-forming apparatus, such as an electrophotographic printer.
2. Description of the Related Art
A fixing apparatus (fixing unit) which heats an unfixed toner image formed on recording paper and thereby fixes the image to the recording paper is installed in image-forming apparatuses such as copying machines and printers, which use electrophotographic recording technology. In general, a heater of a fixing unit generates heat by receiving a supply of power from a commercial AC power source.
In order to satisfy toner image fixing performance, it is necessary to stabilize the temperature of the fixing unit during the fixing process. Therefore, in high-speed image-forming apparatuses in particular, phase control which controls the power supplied to the heater by controlling the conduction angle in the half-wave of the AC waveform, or control which combines phase control and wave number control (hereinafter, called hybrid control) is employed (Japanese Patent Application Publication No. 2011-018027). These types of control have a short control update cycle compared to wave number control, and therefore the power control cycle corresponding to the temperature of the fixing unit can be shortened, which is beneficial for stabilization of the temperature of the fixing unit.
By the way, there are demands to shorten the time period from the inputting of a print instruction, to the outputting of the first sheet of recording paper (the “first print-out time”). One solution for this is a method in which the fixing unit is warmed up by supplying electric power to the heater during standby while waiting for a print instruction. As described above, in an apparatus which supplies power to a heater with a waveform that includes a phase control waveform during printing (during a fixing process), it could be envisaged that power could be supplied with a waveform including a phase control waveform during standby also.
In general, a choke coil is introduced into the heater drive circuit of the fixing apparatus in order to suppress the generation of noise when supplying power to the heater. However, if phase control or hybrid control is employed for heater control, then a humming noise occurs in the coil of the heater drive circuit. Since the humming noise occurs when power is supplied with a waveform including a phase control waveform, then a humming noise occurs also during printing when phase control or hybrid control is used. During printing, the motor, and the like, operates, and the humming noise is not conspicuous because of the sound of these operating parts, but during standby, the operating parts that generate noise are halted and therefore the humming noise of the coil is conspicuous.